not quit enough RPMs

[jvalhenson]

I have a 2004 RG 2100 Sea Chaser(21' 4" with a 9ft beam and pretty heavy for the size) with a 1998 225 Evinrude Ocean Pro. While running WOT with the motor DOWN I am getting 4800 RPMs and 40 mph and WOT with the motor trimmed UP I am increasing to 5100 maybe 5200 rpms and about 45 mph(both measured by GPS.) I know I should be able to get more like 5500-5800 rpms out of this motor. This motor has only been on this boat and has well over 1000 hours on it(all put on it by me) all on this boat with the same prop with no problems as of yet(knock on wood) except for a few electrical/ignition issues but I know I should be getting 300 or 400 more rpms at WOT. The current prop is stainless 14 3/4 x 19. I am not unhappy with the performance of this set-up(cruise at about 35 mph at 4200 rpms getting 2.2-2.3 mpg) but am concerned that running this motor for this long with this many hours(somewhere between 1200 and 1500 hours best I can figure) may be slowly causing some damage although I have seen no evidence of such. Compression comes in between 98 and 103 on all cylinders and she has had had very little done to her in the way of repairs(2 rectifiers, 1 power pack, and a couple sensors that were bad at the start we suspect from just sitting up from 2000 when we bought the motor to 2004 when we bought the boat.) I have been wanting to try out a different prop for a few years now just havent been able to do it yet and really not sure what prop to try. I just cant see a pitch of 19 being to much so I have to lean towards the 14 3/4 diameter being a little to big so thinking of trying a slightly smaller diameter prop but still with a 19 pitch....I dont want to drop to small in diameter though bc it is a very wide boat and I fear that I will have trouble getting up on plane if I get much smaller than whats on it now as at time if we are overloaded in the back it can be a little slow getting up now(jumps on plane quick with a light load.) any ideas on this? Thanks a bunch.

 

[DJ]

Re: not quit enough RPMs

5500+ is your goal.

There is more to prop. selection.

Perhaps:

1. Your engine is too low. Moving 1" up makes a world of differnce.

2. Too high pitch on prop.

I would do # 1 before I start playing with expensive props.

Now that you've "lugged it" for all these years. Healthy does of Sea Foam will help.

 

[jvalhenson]

Re: not quit enough RPMs

We have adjusted the depth of the motor and where it sits now is the best we have gotten in.....thats not to say it couldnt be zeroed in a little better but I believe we have it at its optimum depth. She gets her treatments regularly to keep everything clear.......last year got some bad gas(had several gallons of water in it from the gas station) so we got carb kits and did all the carbs since a lot of that got to the motor before we found out and the old ones still looked brand new....no buildup or anything on em at all...totally clear and had at or over 1000 hours on them then. but like I said I am "pretty" sure the height is right but of course I cannot be totally sure without moving it a little more.

 

[DJ]

Re: not quit enough RPMs

If compression is good, you are over-propped,

Again-5500 is your goal.

 

[SparkieBoat]

Re: not quit enough RPMs

are you saying that you use to get 5800 RPMs (and I assume somewhere around 50 mph) out of this exact same motor on this exact same boat??? if so you need to check all your electronics with DVA test, check timing, check fuel pressure, check engine compression...my 2 biggest suspects would be timer base, or fuel issue.

 

[glengold]

Re: not quit enough RPMs

So you used to get 55-5800 RPM and after several years of use you are only getting 5100?

I'd clear the jets with a piece of wire, check timing, and maybe do a link and sync. The throttle plates all need to open fully.

Make sure the problem isn't cable stretch/slack, if the cable isn't too bad you can pick up some slack with some adjustments.

 

[glengold]

Re: not quit enough RPMs

great minds think alike I see......

 

[SparkieBoat]

Re: not quit enough RPMs

Rofl

 

[jvalhenson]

Re: not quit enough RPMs

no the performance has been exactly the same from the first day. Have never gotten over 5100 maybe 5200 rpms. Throttle opens, timing is right, jets are clear, cables are new and the new ones made no difference over the old ones. I am very sure that the problem is with the prop(although not 100% as there still could be an issue with the motor depth) I just have a hard time believing that a 14 3/4 x 19 prop is to big for this 225 hp motor. Best I can figure is I have the boat weighing around 4600lbs with the motor, loaded down to fish, 4 people, and full of gas. Might try to move it up a little unless I can find a slightly smaller diameter prop to try.

 

[SparkieBoat]

Re: not quit enough RPMs

yes you need to try a 17 pitch prop then...every boat is different, you might get another 21' 4" boat and that prop would work fine..you must prop the motor for the boat. A 17 pitch prop would not be unusual for a heavy deep vee 21' boat, with a 200/225.

 

[glengold]

Re: not quit enough RPMs

I'd also point out that a stainless prop is harder to spin up than an aluminum of the same pitch, so you could try a 19p alum or go to a 17p if you stay with stainless. From what I've heard changing diameter does very little, and most props are made in a particular diameter/pitch combination. you could take the prop to a prop shop and have them take some pitch out of it as well, taking it to a prop shop will probably be the cheapest option.

 

[SparkieBoat]

Re: not quit enough RPMs

a prop shop can only change it about 1" in pitch. he will need a 2" drop in pitch to get his RPMs up to over 5500. the diameter makes about 50-100 RPM difference. every thing is subject to vary because of the number of different style props and manufacturers. it is best if you can find a shop that will let you test a prop and see how it runs on your boat/motor. if you want more hole shot you can get a vented prop..they are very cool indeed.

 

[jvalhenson]

Re: not quit enough RPMs

My spare prop is an aluminum of the same dimensions...havent put it on the boat but might give it a try and see. Hate to take it to a prop shop and change it as it is in very good shape and would rather leave it as is and just get a different prop to try a 17. If they took an inch out of it to an 18 and the performance somehow got worse can than put an inch back in it or once its done its done?

 

[SparkieBoat]

Re: not quit enough RPMs

they can put it back but then you start running up a $$$bill. you can buy a 17 aluminum for under $100

 

[jvalhenson]

Re: not quit enough RPMs

thanks for all the replies guys.....def gonna go with a stainless prop if and when I do swap it out....gotta give a quick nod to ken at propgods for a quick and thorough response to this same question..here was his answer which of course was very similar to the ones here:
"Your numbers are ok, you just have a little too much pitch.

your slip ratio is 11% which is normal (good)

If you drop down to a 17 pitch prop, you'll get stronger acceleration, and probably about the same top speed. Might gain or loose 1 mph but thats about it.
You will cruise a couple mph slower for the same rpm but the engine isn't working as hard. Economy will be about the same, its not going to change much.

Basically, you'll gain acceleration and throttle response throughout the rpm range and not really give up anything."